The present invention relates to detecting the onset of particularly severe corrosion problems and to sensing various elements and compounds in the environment, particularly to sensing environmental conditions which cause corrosion of materials, and more particularly to a corrosion sensor array and monitoring method capable of simultaneously sensing various environmental conditions which cause corrosion of materials and the associated instantaneous corrosion properties/response of structural materials of importance.
Virtually, every structural material in common use is affected by corrosion. For the U.S. economy, the loss of materials due to corrosion carries an annual price tag of tens of billions of dollars. Virtually, all structural materials corrode when placed in liquid or atmospheric environments, particularly in urban/industrial areas. The rates and modes of corrosive attack differ for different materials. Corrosion properties must always be evaluated when selecting materials for use in land vehicles, particularly public transportation, water-craft of all types, aircraft, bridges, buildings, etc. Beside the purely economic factors, there are the even more important issues of human safety which could be jeopardized when structures used for transport and housing are weakened by corrosion.
The rates/modes of corrosive attacks on a given material/metal are related to environmental conditions in which the material/metal is placed. These environmental conditions include such factors as pH and the presence of high concentrations of various ions, such as chloride, sulfide, etc. Therefore, it would be advantageous if a means was available for producing a corrosion sensor which could be embedded in, or emplaced on, various structures and provide an "early warning system" to guard against catastrophic failures of structural materials. Alternatively, such a device could find use in monitoring other environmental conditions, such as acid-rain. In the latter an application, the "detector" portion of the sensor system could be used in a disposable fashion.
Since corrosion is an electrochemical process, sensor systems based on electrochemical methods are well-suited for indicating particularly detrimental corrosive conditions. Various electrochemical sensors (electrodes) are known in the art, as exemplified by U.S. Pat. No. 5,120,421 issued Jun. 9, 1992 to R. S. Glass et al. and document UCRL-JC-106690, "Preparation of Solid Membrane Chloride Ion-Selective Electrodes by Ion Implantation", R. S. Glass et al., February 1991.
There has apparently been very little work directed to the development of corrosion sensors which can be used in-situ. A sensor based upon well-known principles of galvanic corrosion has been designed for atmospheric marine environments, see V. S. Agarwala, "Corrosion Monitoring of Shipboard Environments", ASTM Special Technical Publication 965, S. W. Dean and T. S. Lee, Eds., ASTM, 1986, p. 354-365. A single-element corrosion sensor functioning on the basis of electrical resistance has been designed, see S. T. Stropki et al., Proceedings of the 1989 Tri-service Conference on Corrosion, Report NADC-SIRLAB-1089, V. S. Agarwala, Ed., published by Defense Logistics Agency, Alexandria, Va., 1989, p. 544-561. The electrical resistance method of detection operates on the principle that the electrical resistance of a metallic conductor increases as the cross-sectional area decreases. Thus, one fabricates a device, or deposits a metal of a defined area and geometry and monitors resistance changes between two points. The resistance is related to cumulative corrosion of the metal. The probe can be made from a single material, such as aluminum alloy, 7075-T6. This electrical resistance type sensor was designed for emplacement in difficult to monitor locations on aircraft (helicopters) and proposed for cumulative corrosion monitoring.
A two-element corrosion sensor, consisting of an electrical resistance (ER) probe and an element for linear polarization resistance (LPR) measurement has also been designed, see F. Ansuini, NADC-SIRLAB-1089, p. 533-543. For this sensor, cumulative corrosion is measured by the ER element, while instantaneous measurements of corrosion rates are made using the LPR element. This sensor was proposed for both dry and wet environments, such as in dry and humid air.
A multiple-element sensor has been proposed for evaluating the localized corrosion of stainless steel in bleaching processes, see T. P. Anguish et al., "Development Of A Multiple Element Sensor For Localized Corrosion Of Stainless Steel", NADC-SIRLAB-1089, 1989, p. 562-572. This multiple-element sensor included an electrode of stainless steel, a coated Ag/AgCl reference electrode, a temperature transducer, a Ag/AgCl chloride sensor, and a pH electrode, and by monitoring these parameters the critical regions where pitting is probable can be avoided.
Multielement sensors have not been previously proposed for monitoring corrosion processes which utilize microelectrodes, which have analytical advantages, and which may be used in common sensor arrays.
While various corrosion sensors are known, as pointed above, there is a need for corrosion monitors which can be embedded in, or emplaced on, various structures, or which can be used in environmental monitoring stations, which enables simultaneous measurement and correlation of corrosion properties of structural materials and environmental conditions leading to corrosion. This can be accomplished by using a plurality of individual sensors with each sensor being constructed to detect a different corrosive component or composed of the structural material of interest. The present invention fills such a need by providing a monitoring method utilizing an array of electrodes (sensors), each individual sensor of the array being constructed for detecting elevated concentrations of a specific corrosion-causing ion, parameter, corrosion product, etc., such as measuring a chloride ion, sulfide ion, copper ion, pH, etc., whereby comprehensive and near-simultaneous measurement may be obtained by sequentially accessing the individual array elements. A number of such arrays would be embedded in or emplaced on a structure to be monitored or situated in various environments, and provide an indication of the real-time corrosion behavior of specific structural materials and the environmental factors which dictate the corrosion rates and mechanisms for these materials. Thus, the array of sensors are environmental/corrosion monitors. An inspector can obtain information from the sensors by using a hand-held instrument, or the sensors could be used without operator interface and be deployed in the field with periodic information being telemetrically relayed to a remote data processing facility. Periodic inspections or continuous monitoring are possible, with the preferred mode of operation being apparent from the application.